1. Field of the Invention
The present invention relates to a method of noninvasive measurement of physiologic parameters in a subject. In particular the present invention relates to a method of noninvasive and non-pulsatile measurement of physiologic fluid parameters as well as a combination of nonpulsatile and pulsatile physiologic parameters for a subject, or patient, which may include any animal with body fluid compartments.
2. Discussion of the Related Art
Known art noninvasive methods have been disclosed for determining physiologic parameters such as arterial blood pressure (U.S. Pat. No. 4,178,918), pulse oximetry (U.S. Pat. No. 3,998,550), and vascular compliance (Shankar, U.S. Pat. Nos. 5,241,963 and 5,724,981) using pulsatile signals acquired from the subject. However, these methods are dependent upon and limited by naturally occurring pulsations in the arteries of the subject.
Other known art noninvasive methods have been disclosed for determination of vascular behaviors by rheoplethysmography (Piquard, U.S. Pat. No. 4,169,463) and determination of blood pressure in the veins and arteries (Blazek et al., U.S. Pat. No. 5,447,161) by observing non-pulsating signals acquired from the subject by a technique known as xe2x80x9cvenous occlusion plethysmography.xe2x80x9d This method relies upon a time-based accumulation of blood in the veins of the subject in order to make the physiologic measurements. However, these methods require that the occlusive device be proximal or downstream from the measuring device along the body region of the subject. Also, these methods further lack the ability to calibrate directly to the subject and rely upon coefficients of standardization, which are characteristic of the healthy vascular operation of a normal limb of the same type as the limb under study. In some instances, these methods are limited to application on fingers and toes.
Referencing Table 1 and FIGS. 1-2, the functional parts of the cardiovascular system include the heart, the lungs, the arteries, the capillary beds, the veins, and the blood. The cardiovascular system is comprised of two independent vascular circuits, the pulmonary circuit and the systemic circuit. Each vascular circuit has a system of arteries and a system of veins separated by a capillary bed. Blood is primarily a composite of plasma and red blood cells. The human heart is a four-chamber pump organized as a right half and a left half with two sequential pumping chambers in each half. The upper pumping chamber of the heart is called the atria and the lower pumping chamber of the heart is called the ventricle. The atria serve as filling chambers for the ventricles and contract prior to the time of contraction of the ventricle. This difference in time of contraction allows the atria to pre-load the ventricles prior to the ventricular contraction. The ventricles are the primary pumping chamber for each circuit. The right ventricle pumps blood into the pulmonary circuit and the left ventricle pumps blood into the systemic circuit.
Thus, the right half of the heart supplies blood flow to the pulmonary circuit and the left half of the heart supplies blood flow to the systemic circuit. Each circuit returns blood to the other side of the heart through its associated venous system. The pulmonary circuit is composed of the pulmonary arteries, the lungs, and the pulmonary veins. The pulmonary circuit oxygenates the blood in the lungs and returns the oxygenated blood to the left heart to be pumped into the systemic circuit. The systemic circuit, composed of the systemic arteries, the systemic capillary beds, and the systemic veins, supplies oxygenated blood to all areas of the body. The systemic blood flow is then returned to the right heart by the systemic veins.
The heart produces pulsatile blood flow in the arteries by forcing a volume of blood into the aorta during each contraction of the heart. This volume of blood, known as the xe2x80x9cstroke volumexe2x80x9d, causes an arterial pressure wave to propagate throughout the systemic arteries up to the arterioles (FIG. 1). The arterioles, which are the smallest arteries in the systemic circuit, have the ability to dynamically vary the resistance to blood flow (Table 1) in response to demands from the end cells of the body. This resistance to blood flow is known as the xe2x80x9cperipheral vascular resistancexe2x80x9d. The peripheral vascular resistance causes a reduction in arterial blood pressure as the blood flows through the arterioles into the capillary bed as shown in FIG. 2.
In particular, the arterial blood pressure before the arterioles is highly pulsatile and time variant, while after the arterioles, the arterial blood pressure is primarily steady state, lacking any significant pulsation. Generally, known noninvasive methods of measuring arterial blood pressure, pulse oximetry, and vascular compliance, rely upon the pulsation of arterial blood pressure. As shown in FIG. 2, the blood pressures that exist in the capillary beds and veins after the arterioles, is substantially non-pulsatile and therefore does not lend itself for measurement by known noninvasive pulsatile plethysmographic methods. Furthermore, other physiologic attributes such as oxygen saturation of the blood and compliance of the vessel wall in the non-pulsatile vessels cannot be determined by methods that are reliant on arterial pulsations.
Blood vessels have elastic walls that stretch in response to the volume of blood contained within them. The degree of elasticity or tension of the vessel wall determines how much pressure is produced within a vessel for a specific amount of blood volume or change in blood volume. The blood vessel wall is an active organ with the ability to vary its compliance, or its inverse, wall tension, as discussed below, dependent on sympathetic nervous stimulation to the vessel.
In arteries of the subject, there is both a steady state and a pulsatile time variant volume of blood in the vessel. This is due to the stroke volume of blood forced into the arteries by each contraction of the heart. The increased stroke volume of blood in the arteries caused by the heartbeat creates pulsatile pressure waves that propagate to a point of extinction within the arterial bed. The actual point of extinction of the pulsatile pressure wave can be dependent upon the physiologic state of the arteries of the subject but is generally thought to be in the small arteries and before the arterioles. The amount of arterial pulse pressure change that occurs during each cardiac cycle is dependent on: the elasticity of the arterial wall, the stroke volume produced by the heartbeat, and the peripheral vascular resistance of the systemic circuit.
The elasticity of the vessel is commonly referred to as the xe2x80x9cvascular compliancexe2x80x9d. Vascular compliance is defined as the rate of change of volume in the vessel versus the rate of change of pressure within the vessel. The inverse of compliance is called the vessel wall tension. Compliance and tension are in effect a measure of the level of elasticity or stiffness of the vessel wall. The loss of elasticity of the blood vessel wall contributes to the disease state known as hypertension or elevated blood pressure. It has been extensively reported in medical literature that both a thickening of the blood vessel wall (Atherosclerosis) as well as a plaquing of the interior lining of the blood vessel (Arteriosclerosis) contributes to a reduction in blood vessel elasticity and a general increase in arterial blood pressure. Furthermore, pathologies of the sympathetic nervous system and the adrenal medullae have been shown to dramatically affect the tension or compliance of the blood vessel wall.
The compliance of a vein is generally six to eight times greater than the compliance of an artery. Therefore, veins can accept and hold much larger volumes of blood at lower pressures than arteries. The tension of the vein wall and the volume of blood that it is containing regulate the venous blood pressure. The tension or compliance of the vein is regulated by the sympathetic nervous system. Body fluid volumes are primarily controlled by the function of the kidneys. A device that noninvasively determines the compliance or tension of the compartments throughout the circulatory system would be beneficial in the diagnosis and management of cardiovascular diseases.
Venous blood pressure is of particular interest to healthcare providers because it is a critical parameter in the diagnosis and treatment of a variety of diseases such as heart failure, acute myocardial infarction, pulmonary hypertension, renal failure, and deep vein thrombosis. Furthermore, it would be a valuable physiologic parameter in the triage of emergency care patients for detection of cardiovascular and hemodynamic distress.
Central venous pressure (CVP), also known as the right atrial filling pressure, is of particular interest in diagnosing and managing subjects afflicted with these disease states. Since Venous pressure is a low, substantially non-pulsatile, time invariant physiologic parameter, there has been no prior method demonstrated which directly measures its value noninvasively. Heretofore, CVP has been measured by the expensive and invasive surgical insertion of a transducing catheter into the subject. Therefore, the ability to measure noninvasive central venous pressure with an easy to use noninvasive device would have a substantial impact on the quality and cost of healthcare delivery.
Arterial blood pressure has been measured by both invasive and noninvasive methods in the known art. Invasive blood pressure monitoring requires the insertion of a catheter into an artery for direct transduction of arterial pressure data. Due to the invasive nature of this method, it is considered a separate art and will not be further reviewed here. Two primary known methods of noninvasive arterial blood pressure monitoring, the auscultatory method and the oscillometric method, are discussed below.
The conventional auscultatory method of non-invasive blood pressure (NIBP) monitoring relies upon the sounds made by the blood coursing through a partially restricted artery. A clinician, e.g. a physician, a nurse, a medical technician or a paramedic, wraps a blood pressure cuff around the limb of the subject and inflates the bladder that is built into the cuff to exert on the arm a counter-pressure that exceeds the subject""s systolic blood pressure and consequently occludes the flow of blood through the arteries in the limb. The clinician then slowly releases the cuff pressure while simultaneously observing the pressure indicated by a cuff pressure indicator and listening through a stethoscope for the so-called Korotkoff sound generated within the lower brachial artery. The Korotkoff sound is absent when the bladder counter-pressure is above systolic or below diastolic pressure. The onset of the Korotkoff sound is taken as indicating systolic pressure and the subsequent disappearance of the Korotkoff sound is taken as indicating the diastolic pressure.
The auscultatory method is subject to disadvantage because it requires that a clinician listen for the Korotkoff sound. Further, the precise point of the onset and disappearance of the Korotkoff sound are somewhat subjective and accordingly two clinicians can provide significantly different blood pressure values for the same subject. In addition, unless the clinician releases the cuff pressure slowly in the region of the systolic and diastolic pressures, it is difficult to relate the onset and disappearance of the Korotkoff sound precisely to pressure values. Moreover, the mean pressure cannot be reliably measured directly by the auscultatory method since there is not a definitive sound to identify the mean pressure value. The clinician may estimate the mean pressure from the systolic and diastolic pressures based on knowledge of the typical blood pressure waveform.
The wall of a blood vessel of a subject is elastic and therefore is distended cyclically when pressure waves induced by cardiac activity propagate through the vessel. When an external counter-pressure is applied to a limb of a subject by a cuff bladder circumscribing the limb, the pressure within the bladder is communicated through the tissue bed and fluids of the limb to the wall of the arteries within the limb.
Conversely, the pressure wave within the artery generated by the heart contraction is communicated through the arterial wall, tissue bed and fluids of the limb into the bladder and the fluid in the bladder. Consequently, the pressure of the fluid in the bladder is modulated by the subject""s vascular pressure changes in the limb. These modulations of pressure within the bladder are commonly referred to as oscillations.
The variation in amplitude of the pressure changes induced in the bladder can be measured and is used in the oscillometric method of noninvasive blood pressure monitoring. The amplitude of each oscillation is recorded versus the value of cuff counter-pressure at which it occurred. In the oscillometric method, the cuff bladder is inflated to a pressure above systolic and the cuff pressure is gradually released, as in the case of the auscultatory method, and during the release of cuff pressure the amplitude of pressure variations in the bladder are measured.
The bladder pressure value is filtered, mathematically or electronically or the like, to separate the oscillating and steady-state components of the bladder pressure. The oscillating and steady-state pressure components are amplified and recorded separately. The magnitude of the steady-state component is a measure of the counter-pressure applied to the limb by the bladder while the oscillating component is used to develop the oscillometric complex from which can be determined the mean, systolic, and diastolic pressure values of the subject.
The oscillating component waveform has a characteristic shape, being composed of oscillations at pulse rate within an envelope that initially increases in amplitude then declines to a minimal value, as shown in FIG. 3. The characteristic sequence of oscillations of increasing and decreasing peak-to-peak amplitude versus counter-pressure is known as the oscillometric complex. Since the maximum change in bladder pressure occurs when the counter-pressure is equal to the mean blood pressure, the mean blood pressure is equal to the counter-pressure at which the pressure wave oscillations reach their maximum amplitude.
The maximum peak-to-peak amplitude is multiplied by a derived factor X to define a systolic detection amplitude and by a derived factor Y to define a diastolic detection amplitude. A search algorithm examines the oscillations of the oscillometric complex, starting from the mean detection point and moving towards higher counter-pressure, to identify the first pulse of peak-to-peak amplitude less than the systolic detection amplitude and defines the occurrence of this pulse as the systolic detection point. Similarly, the search algorithm examines the pulses of the oscillometric complex, starting from the mean detection point and moving towards lower counter-pressure, to identify the first pulse of peak-to-peak amplitude less than the diastolic detection amplitude and defines the occurrence of this pulse as the diastolic detection point. The counter-pressure associated with systolic detection point is reported as the systolic pressure and the counter-pressure associated with the diastolic detection point is reported as the diastolic pressure. The oscillometric method thusly determines the mean blood pressure and estimates the systolic pressure and the diastolic pressure.
The advantage of oscillometry, i.e. the oscillometric method, is that it is easily automated and can serve to take repeated intermittent blood pressure measurements over relatively short periods of time, such as once per minute.
Body Fluid Compartmentsxe2x80x94Volume and Status Monitoring
Fluids within the body of a subject are referred to herein as compartmentalized in vascular and nonvascular, and in some instances extracellular fluid and intracellular fluid, compartments. The extracellular fluid compartment contains all fluids that exist outside of cells and include interstitial fluid, blood, plasma, lymph, cerebrospinal fluid, intraocular fluid, gastrointestinal fluid, and fluids of the potential spaces. Intracellular fluids are all fluids contained within cells of the body including the red blood cells. In general, 60% to 70% of the total body fluid in a subject is contained within the cells of the subject with the remaining fluid being contained in extracellular fluid compartments such as the various vascular vessel types, each of which is deemed a different compartment for purposes of explanation herein. Understanding the distribution of body fluids in each vascular, nonvascular, intracellular or extracellular fluid compartment can be indicative of disease states of the subject and means and method for doing so would be a desirable addition to the art.
A plethysmograph measures a change in volume and records the changing value. Plethysmography has been utilized in physiologic monitoring to measure the respiration rate, the oxygen saturation of blood, and the pulse volume in arteries. Various methods of plethysmography have been provided in known art including volume plethysmography, impedance plethysmography (e.g., Shankar, U.S. Pat. Nos. 5,241,963 and 5,724,981), which are forms of pulse plethysmography and venous occlusion plethysmography (e.g., Piquard, U.S. Pat. No. 5,169,463, supra.). As evidenced in the art, the plethysmograph, independent of sensing modality, measures a time rate of increase or decrease in volume within the subject.
Volume plethysmography is a method of noninvasive physiologic measurement described by Shankar for measuring the volume of arterial pulsations in the limbs of a subject. Shankar provides a method of measuring pulse volumes from the arteries of subjects and relating that information to the state of atherosclerosis of the arteries. This plethysmographic method is reliant upon arterial pulsations generated within the body of the subject and therefore is incapable of measuring the physiologic characteristics of vessels of the body which do not contain pulsatile flow.
Venous occlusion plethysmography is a method of noninvasive physiologic measurement reliant upon the accumulation of fluids in the veins of a body region that is distal or upstream from a venous occlusion device. Reference for Venous Occlusion Plethysmography is made to xe2x80x9cPrinciples of Applied Biomedical Instrumentationxe2x80x9d, 3rd Ed, L. A. Geddes, L. E. Baker, John Wiley and Sons, 1989. The venous occlusion device must be capable of applying a pressure that is greater than the venous pressure and less than the arterial pressure to the body region in order to cause a time variant accumulation of venous blood in the upstream-monitored body region of the subject. It will be noted that xe2x80x9cupstreamxe2x80x9d is determined according to the area of higher fluid pressure, and in this instance as relative to the occlusion, although upstream may be the physically distal position of the body limb.
Because venous occlusion plethysmography relies upon the accumulation of fluid in the upstream body region of the subject (again relative to the occlusion), the method is limited to closed body regions such as limbs, legs, arms, fingers, and toes. Physiologic parameters are measured by plethysmographic methods from the accumulating fluids during the period of venous occlusion. A problem with this method is that the physiologic measurements are made on a body region that is being altered from its natural state prior to determination of the desired physiologic parameters. As abnormal fluid volumes accumulate in the fluid compartments of the upstream body region, fluid pressure increases above normal and venous compliance decreases below normal. These abnormal conditions can introduce errors and uncertainty in the physiologic measurements.
Impedance plethysmography is the measurement of impedance changes in a subject by measuring the time variant electrical impedance of the subject and determining volume characteristics based upon measured impedance values. Known impedance plethysmographic methods have largely been limited to the measurement of impedance changes caused by pulsatile, or time varying, volume changes in the arteries of the subject and to venous occlusion plethysmography which depends upon a separation between the pressure applying device and the impedance sensing device.
The ability to non-invasively measure volumetric changes in the vascular bed by impedance plethysmography has been extensively discussed in the literature including Nyboer, J., xe2x80x9cElectrical Impedance Plethysmography: A Physical And Physiologic Approach To Peripheral Vascular Study.xe2x80x9d Circulation, 2:811-821, 1950; Geddes, and Sadler, xe2x80x9cThe Specific Resistance Of Blood At Body Temperature.xe2x80x9d xe2x80x94Med. Biol. Eng. 11(3):336-339, 1973; Shankar, Webster, and Shao, xe2x80x9cThe Contribution of Vessel Volume Change and Blood Resistivity Change to the Electrical Impedance Pulsexe2x80x9d, IEEE Transactions on Biomedical Engineering, Vol. BME-32, No. 3, Mar. 1985; Chumlea, Guo, Baumgartner, and Siervogel, xe2x80x9cDetermination Of Body Fluid Compartments With Multiple Frequency Bioelectric Impedance.xe2x80x9d xe2x80x94Human Body Compositionxe2x80x94In Vivo Methods, Models, and Assessment, Plenum Press, Basic Life Sciences Vol. 60; Shimazu, Yamakoshi, Togawa, and Fukuoka, xe2x80x9cEvaluation Of Parallel Conductor Theory For Measuring Human Limb Blood Flow By Electrical Admittance Plethysmographyxe2x80x9d, January 1981, IEEE Transactions on Biomedical Engineering; xe2x80x9cEncyclopedia of Medical Devices and Instrumentationxe2x80x9d, Volume 3, pg. 1633, 1988, John Wiley and Sons, New York; Nyboer, xe2x80x9cElectrical Impedance Plethysmography,xe2x80x9d 2nd Edition, Thomas Books, Springfield, Ill., 1970; and Lifshitz, xe2x80x9cElectrical Impedance Cephalography, Electrode Guarding And Analog Study,xe2x80x9d Ann. N.Y. Acad. Sci. 170:532-549, 1970.
Impedance monitoring is based upon the relationship E=I*R, known as Ohm""s Law, which states that the voltage drop (E) across a length of conductor is equal to the current flowing through the conductor (I) times the resistance of the conductor (R). When the current is an alternating current (AC), the resistance element becomes a vectorial sum of a real component (R) as well as an imaginary component (jX) and is referred to as impedance. The complex nature of impedance arises from the presence of capacitance or inductance in the conductive material. The impedance (Z) of a conductor is represented as a complex number (Z=R+jX) composed of the resistance (R) and reactance (jX) of the conductor. Furthermore, the admittance (Y) of a substance is equal to the inverse of the impedance or 1/Z of that substance. The phase angle ( ) of impedance (Z) is the TANxe2x88x92(jX/R).
Nyboer was the first to apply the formula for the resistance of a homogeneous volume conductor to predict the relationship between impedance changes and blood volume changes. The electrical impedance (Z) of a body region (b), can be expressed in terms of its cross sectional area (acs), length (L), and resistivity ( ) of the material.
Zb=bL/acsxe2x80x83xe2x80x83(1) 
acs=bL/Zbxe2x80x83xe2x80x83(2) 
acs=bLYbxe2x80x83xe2x80x83(3) 
Since the volume of a vessel segment is Vb=L*acs, then the electrical impedance can be expressed in terms of the segmental or regional volume which can be expressed in terms of the electrical impedance as shown in equations 4 and 4a respectively.
Zb=bL2/Vbxe2x80x83xe2x80x83(4) 
Vb=bL2/Zbxe2x80x83xe2x80x83(4a) 
Nyboer further assumed that the segmental blood volume change (V) could be modeled as the impedance change (Z) due to the change in blood volume electrically in parallel with the basal tissue impedance Zt (see FIG. 4). This led to the well-known xe2x80x9cNyboer Formula,xe2x80x9d
Vb=xe2x88x92bL2Zb/Zt2xe2x80x83xe2x80x83(5) 
This relationship between the change in volume of blood and its associated change in impedance is the basis of impedance plethysmography. The Nyboer formula has been widely practiced in the known art with mixed results.
The Nyboer formula is dependent upon a changing impedance value (Zb) in order to determine a changing volume (Vb) within the body of the subject. Further, the knowledge of the specific value of b for the particular subject is critical to the accuracy and reliability of the volume data determined by the Nyboer formula. It is further known that the resistivity of blood is dependent on the hematocrit of the particular subject and therefore will vary from subject to subject.
In the Nyboer formula (5), it is assumed that all elements are constant except Zb indicating the pulsatile volume changes in the arteries. This assumption is only true if there are no other concurrent material volume changes occurring in the monitored body region of the subject. However, in practical applications of impedance plethysmography, this assumption is often not true and therefore the Nyboer formula can often generate unreliable and inaccurate results due to respiration and motion artifacts in the impedance data.
Further, the Nyboer formula assumes a composite resistivity for the diverse elements that comprise Zt. This composite resistivity may vary from subject to subject and contribute uncertainty to the volume values derived using the Nyboer formula. It is important to note that even though the Nyboer formula has been widely used in the measurement of physiologic parameters, the Nyboer methodology has been criticized by some practitioners as being unreliable and inaccurate.
It is generally known that a body region of the subject is composed of a variety of conductive materials including fluids, tissue, fat, and bone. It is further known that cell membranes and vessel walls surrounding fluids may affect the conductivity of the cells and vessels. Therefore, a body region of the subject is considered a composite conductive medium with multiple current pathways through the body region. Each conductive pathway is considered to have unique electrical conductivity characteristics. The xe2x80x9cParallel Conductor Theoryxe2x80x9d describes the subject as a parallel conductor model composed of various conductive elements that represent different materials of the subject""s body composition. The parallel conductor hypothesis models a body region as a set of parallel conductors in which the volume of arterial blood in the body region is the only time variant conductor in a compliant subject.
The parallel conductor model shown in FIG. 5 is composed of a constant admittance value (Yt) that represents the composite parallel conductance characteristics of all the non-changing conductive pathways in the body region, and a pulsed changing value (Ybv) that represents the pulsatile or time changing volume of arterial blood in the body region. Yt represents the total admittance value of all non-changing elements in the body region including fat, bone, lean tissue, extracellular fluid, intracellular fluid, and nonpulsatile blood volumes in the body region. Ybv represents the admittance of the variable arterial blood volume. This model is consistent with the simplified Nyboer formula (FIG. 4).
However, the parallel conductor model may also be expressed to show the total admittance of a body region by modeling each conductive element in the body region as parallel conductors:
YT=Ybv+Ybc+Yvc+Yof+Ytis+Ybone+Yfat (admittance form)xe2x80x83xe2x80x83(6a) 
Or the total impedance of the body region by parallel impedances:
1/ZT=1/Zbv+1/Zbc+1/Zvc+1/Zof+1/Ztis+1/Zbone+1/Zfatxe2x80x83xe2x80x83(6) 
In a parallel impedance network in which the blood is a single element of a complex system of conductive elements, it would be valuable to be able to isolate the affects of the various impedance elements on the measured composite impedance value. Applicants do not believe that the known art has taught this valuable principle.
Photo-Plethysmography is a known method of measuring the characteristics of blood and tissues by the amount of absorption of particular wavelengths of light. See for example U.S. Pat. No. 5,447,161 to Blazek et al, which combines venous occlusion and photo plethysmography techniques.
To applicants"" knowledge, none of the aforementioned known art has recognized that information on the entire vascular system may be gained by combining plethysmography with the depletion or replenishment of body fluids under controlled conditions.
Upon gaining an understanding of the foregoing discussion, the person having ordinary skill in the art will appreciate that the circulatory system, in all its components, is connected, interactive, interdependent and largely unexplored or monitored outside of its large or pulsatile, or both, components, such as the heart and large veins and arteries. Therefore what is needed are means and methods for gathering physiologic data on each part of the circulatory system for gaining further knowledge of the operation of the circulatory system, including each vascular compartment or chamber or vessel type, including the large arteries, small arteries, arterioles, capillaries, venules, small veins and large veins. It is further desirable that such means be non-invasive, easily utilized and inexpensive when compared to the high cost of invasive techniques.
The present invention provides means and methods for gathering physiological parameters, or data, on each part of the circulatory system, thus affording opportunity for gaining further knowledge of the operation of the circulatory system, including each vascular compartment or body fluid chamber, or vessel type, including the large arteries, small arteries, arterioles, capillaries, venules, small veins, large veins and nonvascular fluids. It is the inventors"" belief that as such means provide for increased knowledge of the parts and behavior of the circulatory system, other fluid compartments or physiologic data of significance may be identified. The present invention provides such means in a non-invasive manner which is easily utilized and is inexpensive when compared to the high cost of invasive techniques.
The present invention provides means and methods for noninvasively identifying the blood pressure characteristics in each of the seven types of vessels in the circulatory system, including the Central Venous Pressure (CVP), through a single monitoring system. According to certain aspects of the invention, a known pressure is applied to a body region in increasing amounts to force blood volume depletion from the body portion in step-wise fashion through each vessel type. Nonvascular fluid compartment parameters may also be identified. Fluid volume depletion, or replenishment, or both, for each compartment type is then tracked, or measured, in the pressurized area, e.g. by the increasing electrical impedance of the body part during depletion, or by other plethysmography techniques; and is plotted against the increasing pressure data. Pressure and volume data are then referenced against one another, such as by graphing or otherwise recording the data. Such a graph may be done virtually, e.g., mathematically, without the plotting of an actual curve. Release of the applied pressure may then yield similar data by measuring fluid volume replenishment, as indicated by decreasing impedance, against the decreasingly applied pressure during the same diagnostic session.
In the case of an actual graphing, the resulting series of slope changes witnessed within the plotted curve can be seen to reveal the characteristic blood pressure state in the range between each slope change, for each vessel type placed under depletion pressure. In the case of the virtual graphing, the apparatus may mathematically identify the points at which a coordinate linear, or curvilinear, relationship changes for the body fluid volume indications as referenced to the series of pressure values. That is, by detecting the change in the slope of the virtual graph, the state transitions, i.e. transitions from one pressure state to the next, are indicated in the vascular fluid profile between two adjacent vessel types (i.e. the transition points between vascular types).
The method is not dependent on oscillometric or pulsatile measurement methods. The measurement area for the volume sensor and the area of uniform pressure application to the body part are coextensive to ensure accurate indication of fluid volume depletion and replenishment and may be incorporated into a single integrated structure.
The present invention is not limited to the measurement of physiologic parameters in the large, and (to some minor extent) the small arteries. The present invention""s lack of dependency on sensing the natural pulsations of the subject eliminates the time dependency on the occurrence of the naturally occurring pulsation for the measurement of physiologic parameters and allows for determination of physiologic parameters in fluid compartments that are non-pulsatile or oscillometric in nature. The present invention further reduces or eliminates the confounding signals of motion artifact and respiration in the acquisition and processing of the acquired physiologic data.
The present invention overcomes limitations of venous occlusion plethysmography to identify various physiologic properties of the subject in non-venous vessels of the body such as capillary vessels, arteriolar vessels, and nonvascular fluids. The present invention improves the capability of noninvasive pressure and volume measurement instruments to observe the characteristic physiologic behaviors of the fluid compartments of the subject during the depletion of fluid, during static fluid, and during replenishment of fluid, in each of the fluid compartments of the body region of the subject. Furthermore, the present invention is not limited to use on an appendage of the body of the subject but may be applied to any body region of the subject containing body fluid compartments.
xe2x80x9cTransition Pointsxe2x80x9d, xe2x80x9cstate transitionsxe2x80x9d, and xe2x80x9cstate changesxe2x80x9d, are used synonymously to indicate a change from one pressure state (indicative of a particular fluid pressure and hence a particular vessel type according to aspects of the present invention) to a different pressure state.
xe2x80x9cKnown pressure valuexe2x80x9d as used herein includes known or measured values at the time of force application.
xe2x80x9cLinear relationshipxe2x80x9d as used herein includes curvilinear relationships for ease and simplicity of explanation, unless otherwise noted.
xe2x80x9cGraphingxe2x80x9d as used herein includes any physical or virtual representation or construct referencing one type of value against another type of value, unless otherwise noted.
xe2x80x9cUpstreamxe2x80x9d is used herein in the sense of an area of higher fluid pressure, while xe2x80x9cdownstreamxe2x80x9d indicates an area of lower fluid pressure.